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Low Thermal Expansion Ceramic Having High Fracture Toughness

IP.com Disclosure Number: IPCOM000119684D
Original Publication Date: 1991-Feb-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 3 page(s) / 117K

Publishing Venue

IBM

Related People

Clarke, DR: AUTHOR

Abstract

This new class of ceramic materials combines a low and controllable thermal expansion coefficient with high fracture toughness. These materials are based on the incorporation of tetragonal phase zirconia into a small grain matrix ceramic having the pseudo-brookite crystal structure. One particular material in this class would be tetragonal zirconia (stabilized with titanium dioxide) in a matrix of aluminum titanate (Al2Ti5). In addition to the transformation toughening afforded by the presence of the tetragonal zirconia, the aluminum titanate provides the opportunity to have microcrack toughening if properly engineered.

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Low Thermal Expansion Ceramic Having High Fracture Toughness

      This new class of ceramic materials combines a low and
controllable thermal expansion coefficient with high fracture
toughness.  These materials are based on the incorporation of
tetragonal phase zirconia into a small grain matrix ceramic having
the pseudo-brookite crystal structure.  One particular material in
this class would be tetragonal zirconia (stabilized with titanium
dioxide) in a matrix of aluminum titanate (Al2Ti5).  In addition to
the transformation toughening afforded by the presence of the
tetragonal zirconia, the aluminum titanate provides the opportunity
to have microcrack toughening if properly engineered.

      Although the thermal expansion coefficients of most ceramic
materials are small relative to those of metals, they are typically
non-zero and, in particular, are generally larger than that of
silicon.  This simple fact has two major consequences, one that is
quite general and one that is specifically related to their use as
substrates for silicon chip carrying.  The general consequence of
having a non-zero TCE is that when a polycrystalline ceramic is
subject to a sudden change in temperature (so-called thermal shock),
it develops internal stresses because of the relatively poor thermal
conductivity of these materials.  In particularly severe thermal
shock situations these thermally induced stresses can be so great as
to cause the ceramic to fracture.  The magnitude of the thermal shock
stresses is dependent on a number of factors.  In addition to sample
geometry effects, the stresses depend on a combination of material's
properties, including TCE, Young's modulus, thermal conductivity, and
heat transfer coefficient.

      For silicon chip carrier (substrate) applications, the
consequences of the ceramic having a different TCE from the silicon
(thermal expansion mismatch) is that, as the temperature is changed,
for instance during the C4 connection process and during use, such as
turning the chips off and on, thermal stresses are generated.  These
can be manifested in a number of ways, such as to subjecting the C4
solder balls to a cyclic stress that can lead to fatigue failure of
the C4.

      The mineral pseudo-brookite, Fe2TiO5, is the prototype of a
number of orthorhombic compounds that possess an unusually large
crystallographic variation in linear thermal expansion coefficient.
Some of the principal compounds and their axial thermal expansion
coefficients are listed in Table 1.  Although this property of these
compounds has been known for many years they have not found wide
application since materials m...